In a conventional radiography system, an x-ray source is actuated to direct a divergent area beam of x-rays through a patient. A cassette containing an x-ray sensitive phosphor screen and light and x-ray sensitive film is positioned in the x-ray path on the side of the patient opposite the source. Radiation passing through the patient's body is attenuated in varying degrees in accordance with the various types of tissue through which the x-rays pass. The attenuated x-rays from the patient emerge in a pattern, and strike the phosphor screen, which in turn exposes the film. The x-ray film is processed to yield a visible image which can be interpreted by a radiologist as defining internal body structure and/or condition of the patient.
More recently, digital radiography techniques have been developed. In digital radiography, the source directs x-radiation through a patient's body to a detector in the beam path beyond the patient. The detector, by use of appropriate sensor means, responds to incident radiation to produce analog signals representing the sensed radiation image, which signals are converted to digital information and fed to a digital data processing unit. The data processing unit records, and/or processes and enhances the digital data. A display unit responds to the appropriate digital data representing the image to convert the digital information back into analog form and produce a visual display of the patient's internal body structure derived from the acquired image pattern of radiation emergent from the patient's body. The display system can be coupled directly to the digital data processing unit for substantially real time imaging, or can be fed stored digital data from digital storage means such as tapes or discs representing patient images from earlier studies.
Digital radiography includes radiographic techniques in which a thin spread beam of x-rays is used. In this technique, often called "scan (or slit) projection radiography" (SPR), a spread beam of x-rays is directed through a patient's body. The beam is scanned across the patient, or the patient is movably interposed between the beam x-ray source and an array of individual cellular detector segments. In such an embodiment, relative movement is effected between the source-detector arrangement and the patient's body, keeping the detector aligned with the beam, such that a large area of the patient's body is scanned by the beam of x-rays. Each of the detector segments produces analog signals indicating characteristics of the received x-rays.
These analog signals are digitized and fed to the data processing unit which operates on the data in a predetermined fashion to actuate the display apparatus to produce a display image representing the internal structure and/or condition of the patient's body.
One of the general advantages of digital radiography is that the digital image information generated from the emergent radiation pattern and incident on the detector can be processed, more easily than analog data, in various ways to enhance certain aspects of the image, to make the image more readily intelligible and to display a wider range of anatomical attenuation differences.
Recent advances in digital radiography have given rise to techniques known as "energy subtraction" in which tissue specific images can be made, wherein the image of only a particular type of tissue, e.g., bone or soft tissue, dominates the picture. Such techniques are described in an article by Lehmann, L. A. et al.: "Generalized Image Combination In Dual KVP Digital Radiography", Medical Physics, 8: 659-667, 1981, which is expressly incorporated herein by reference.
It has been proposed in energy subtraction to utilize a particular type of dual energy detector assembly which can produce separate signals representing each of lower and higher x-ray energy incident on the detector. Such a detector assembly enables the practice of energy subtraction without the necessity for switching KVP x-ray output levels, or employing other means for periodically attenuating the x-ray beam, such as rapid interposition and removal of a filter to and from the x-ray path. Such a detector employs a dual layer of phosphor-detector elements wherein the phosphor material of a first, or front layer preferentially responds to energy of a relatively lower energy value. A second, or rear, detector layer preferentially responds to x-ray energy in a higher range. Such a detector, and its method of use, is described in U.S. patent application Ser. No. 444,605, filed Nov. 26, 1982 by Gary T. Barnes, which application is hereby expressly incorporated by reference. Such a detector is also described in corresponding published European patent application No. 83307157.4 published on Aug. 8, 1984 by Gary T. Barnes, also incorporated by reference.
A digital radiation SPR imaging system as generally described above is explained in the following publication: Tesic, M. M. et al., "Digital Radiography of The Chest: Design Features And Considerations For a Prototype Unit", Radiology, Vol. 148 No. 1, pages 259-264, July 1983, which publication is hereby expressly incorporated by reference.
It has been proposed to provide the scanning motion in slit projection radiographic systems by means of electromechanical servo-systems driven by controllable electric motors. In such systems, an encoder is utilized to provide a closed loop feedback system wherein motor performance is adjusted in accordance with the sensed location of the detector.
While slit projection radiographic systems of this type have been tested and found satisfactory for operation in permanent installations, such as in permanent doctors' offices and large hospitals, these systems are inordinately complex and bulky for convenient use in portable applications. Such portable applications can include mobile x-ray equipment for transport to a scene of traumatic injury, such as for use in connection with domestic trauma treatment centers, and in mobile military hospitals and first aid stations.
In such applications, it is particularly desirable that all equipment be as simple and reliable as possible, since repair capability may be inaccessible in the field. The equipment should be able to withstand repeated assembly and disassembly for transport. It must be capable of being "knocked down," in such disassembly, into relatively small components which can be carried by humans without the aid of mechanical lifting and transport equipment, such as where it would be desirable to load an x-ray system in pieces into a helicopter for quick transport to and reassembly at a site of need.
Needless to say, x-ray equipment designed for mobile application must be sufficiently rugged to resist damage or maladjustment resulting from vibration and other shock which normally occurs during transport of field equipment.
Another problem inherent in mobile x-ray equipment is that, often, the equipment is used where electric power is in limited supply and form. It is sometimes a problem to find sufficient electric power, or the needed frequency, phase and/or voltage, to actuate relatively heavy electromechanical components such as motors and other servo equipment used to drive prior art type radiographic scanning equipment.
The requirements of radiographic equipment used for initial evaluation of extensive traumatic injury often differ somewhat from the requirements for radiographic equipment used in permanent installations. Often, in mobile units such as military field hospitals, often called "MASH", the most important requirement for a radiographic system is to be able to reliably scan large areas of the human body very quickly, and to rapidly produce an image of reasonable quality illustrating gross traumatic injury caused by shrapnel, bullets and the like. Extremely high degrees of resolution are not as important where the injuries sought to be diagnosed are generally large and easy to identify, given an image of reasonable quality.
It is an object of this invention to provide a lightweight, rugged, reliable, simple, inexpensive, easily disassembled moderate resolution digital slit projection radiographic system capable of executing scanning at an approximately constant velocity, without the need for the application of electromechanical scanning power.